Two Northwestern Williston Basin field studies illustrate the cost-effective geological and three-dimensional seismic methods used to explore for structurally controlled oil reservoirs in the Tule Creek area of northeast Montana. These examples present the geologic and seismic characteristics of a Mississippian Charles C zone pool at Northwest Poplar field and a Devonian Nisku accumulation at Long Creek West field.A classic, upward-shoaling, nearshore marine carbonate sequence characterizes the reservoir and associated strata in both fields. The Charles C zone pool of Northwest Poplar field is a subtle, tectonically-controlled accumulation which remained undiscovered and downdip from the large, nearby, well-known East Poplar field until 1983. Long Creek West field exhibits the subtle character of small closed structures produced by multistage evaporite solution.Three-dimensional (3-D) seismic techniques best detect such fields of low vertical relief and small areal extent. The significant acquisition costs of multifold, 3-D seismic surveys led us to design, acquire, process, and interpret single-fold 3-D surveys to disclose these subtle, small structures. Augmented conventional 48-channel vibroseis crews collected the 3-D data. Near-surface glacial till deposits in the area created residual statics and normal-moveout problems on the single-fold data. These processing problems were handled by constraining the statics and normal-moveout velocity solutions with plausible geologic conditions: the near-flat attitude of the reflecting beds, and the inferred horizontal simplicity of the earth velocity field. Our geologically constrained processing methods included: single-event correlation picking, single-fold, surface-consistent statics, and velocity interpolation on flattened events. In addition, spatially oriented six-fold stacking increases the signal-to-noise ratio in areas where poor signal quality exists. In the case of Northwest Poplar field, geologic studies and subsequent interpretations involving isochron mapping techniques showed that the absence of Mississippian evaporites and paleostructural thinning produce disruption of evaporite events and interval thinning near the evaporites, respectively. Long Creek West field exhibited the interval thickening below, and the interval thinning above the trap formed by two-stage evaporite solution.Careful analysis of geologic conditions coupled with single-fold 3-D methods makes it possible to economically explore for small undiscovered structural traps in a mature exploration area.